A Transmission Protocol like Stream Control Transmission Protocol (=SCTP) transmits packets to the remote peer with a size equal or smaller than a Maximum Transmission Unit (=MTU). In case of SCTP the MTU of a path can be discovered utilising the path-MTU discovery algorithm of RFC1191 (or similar techniques). However the MTU of a certain path within a SCTP association can change over time. If the path-MTU (MTU of a certain path) grows during a SCTP association then there is no problem. If the path MTU decreases, then packets with size equal to the old path MTU will not be able to be delivered to the remote peer as the link or router with the smaller MTU will drop the packet. Thus all such packets will remain unacknowledged and will lead to head of line blocking for in-sequence delivery of packets of the streams to which to non-delivered packets belong. This leads to the failure of the association.
The problem has not been solved in SCTP up till now.